Sincerely, I have a lot of doubts regard of the usage of spatial reference identifier (SRID). I chose 4326 because It's suggested in many documents :-P , then, for calculating the Italy's area, I chose 26591 as I saw in spatial_ref_sys table that it is related to that country and has meters as unit of measurement.
Calculating every country's area with this last SRID yields big gaps respect to the values in the shapefile.
Otherwise, using directly 4326 results, for Italy's area, in a small 33.1005615782892: